Antibiotics: mode of action and mechanisms of resistance. Slides made by Special consultant Henrik Hasman Statens Serum Institut

Transcription

1 Antibiotics: mode of action and mechanisms of resistance. Slides made by Special consultant Henrik Hasman Statens Serum Institut

2 This presentation Definitions needed to discuss antimicrobial resistance Classes of antimicrobial drugs Targets for antimicrobials Mechanisms of resistance Exercises

3 What is the difference between antibiotics and antimicrobial agents?

4 Antibiotics: Naturally occurring microbial products Antimicrobial agents: Any agent used to treat systemic infections Any agent used for disinfection Any compound used as an antiseptic agent

5 Can anybody name an antibiotic? Can anybody name an antimicrobial agent?

6 70% ethanol is a product from microorganisms and kills bacteria. Is ethanol an antibiotic? NO! Antibiotics should not be harmful to the host in the concentrations used for treatment! Furthermore, they should be able to enter the site of infection in therapeutic concentrations.

7 Why does bacteria produce antibiotics? Chemical warfare This gives the antibiotic-producing organism a growth advantage in its niche

8 Antibiotic Antibioticproducing organism

9 Staphylococcus aureus (bacterium) Penicillium chrysogenum (fungus) Zone where bacterial growth is inhibited

10 Antibiotics: The short version Class Aminoglycosides Cephalosporins Macrolides Penicillins Phenicols Quinolones Rifamycins Sulfonamides Tetracyclines Origin/organism Streptomyces, Micromonospora sp Cephalosporium sp Various Actinomycetes Penicillium sp Streptomyces venezuelae* Synthetic Amycolatopsis mediterranei Synthetic Streptomyces sp

11 Mechanisms of antibiotics I Bacteriostatic Stops growth of the infectious agent but does not kill it The immune system has to kill the bug Bactericidal Actively kills the infectious agent (some only growing bacteria)

12 Bacteriostatic antibiotic classes Tetracyclines Aminoglycosides (Gentamicin, Apramycin, Neomycin, Spectinomycin, Streptomycin) Sulphonamides (Sulphamethoxazole) Macrolides (Erythromycin) Amphenicols (Chlorphenicol, Florphenicol) Trimethoprim Polymoxins (Colistin)

13 Bactericidal antibiotics classes Beta-lactams Penicillins (ampicillin, methicillin) Cephalosporins (Cefotaxime, Ceftazidime, Ceftiofur) Monobactams (Aztreonam) Carbapenems (Imipenem, Meropenem, Erthapenem) Quinolones (Nalidixan) Fluoroquinolones (Ciprofloxacin) Glycopeptides (Vancomycin)

14 Spectrum? Small spectrum Only kills a small sub-set of bacterial species (e.g. Strep s) Broad spectrum Kills many different bacterial species including G-ve s

15 Spectrum of antibiotics

16 Which processes does antimicrobial agents interfere with in bacteria?

17 To kill a bacteria, an antimicrobial agent should hit vital processes in bacteria. Can anybody name at least one such process?

18 Antibiotics: Modes of action Inhibitors of DNA synthesis Inhibitors of bacterial protein synthesis Inhibitors of bacterial cell wall synthesis

19 From DNA to protein RNA polymerase Nucleotides A G Ribosome T C RNA polymerase

20 Bacterial growth

21 A bacterial cell wall is composed of a macromolecule of peptidoglycan composed of NAG-NAM chains that are cross-linked by peptide bridges between the NAM subunits. New NAG and NAM subunits are inserted into the wall by enzymes, allowing the cell to grow. Normally, other enzymes link new NAM subunits to old NAM subunits with peptide cross-links. Growth Normal bacterial cell wall NAM NAG Cross-link NAG-NAM chain NAG = N-acetylglucosamine NAM = N-acetyl muramic acid

22 Figure 10.3d The effect of penicillin on peptidoglycan in preventing NAM-NAM cross-links Penicillin interferes with the linking enzymes, and NAM subunits remain unattached to their neighbors. However, the cell continues to grow as it adds more NAG and NAM subunits. The cell bursts from osmotic pressure because the integrity of peptidoglycan is not maintained. Growth New NAM-NAM cross-links inhibited by penicillin Previously formed cross-links remain unchanged

23 Inhibition of cell wall synthesis Penicillin

24 Inhibition of Protein Synthesis Prokaryotic ribosomes are 70S (30S and 50S) Eukaryotic ribosomes are 80S (40S and 60S) Drugs can selectively target translation Mitochondria of animals and humans contain 70S ribosomes Therefore, antimicrobials interacting with the 70S can be harmful if they are able to penetrate the host (eukariotic) cells

25 Antimicrobials that inhibit protein synthesis 30 S subunit Aminoglycosides such as streptomycin and gentamicin Tetracyclines 50 S subunit Chloramphenicol, lincosamides, streptogramins, and macrolides such as erythromycin

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27 Inhibition of Nucleic Acid Synthesis Quinolones (Naldixic acid) and fluoroquinolones (Ciprofloxacin) Act against prokaryotic DNA gyrase (part of the DNA replication machinery) DNA gyrase (Fluoro-) quinolones DNA 2012 Pearson Education Inc.

28 Antibiotics Cell wall synthesis β-lactams: Penicillin Cephalosporins Monobaktams Carbapenems Vancomycin Anti metabolittes Sulfonamides Trimethroprim DNA replication Fluoquinolones Metronidazole RNA-polymerase Rifampicin Protein syntese Inhibitors (50s) Macrolides chloramphenikol Clindamycin Cell membrane Polymyxin Amphotericin Protein syntese Inhibitors (30s) Tetracyclines Aminoglykocides Fucidinic acid

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30 How do we measure the effect of an antimicrobial agent against a given pathogen?

31 Dilution method MIC

32 Question How can we discriminate between biocidal and biostatic antimicrobial agents in a MIC experiment?

33 MIC results Wild type population Name MIC Isolate 1 1 mg/ml Isolate 2 2 mg/ml Isolate 3 4 mg/ml Isolate 4 2 mg/ml Isolate 5 2 mg/ml Isolate 6 4 mg/ml Isolate 7 1 mg/ml Isolate 8 2 mg/ml Isolate 9 1 mg/ml Isolate 10 2 mg/ml Isolate 11 2 mg/ml MIC distribution of Wild type bacterial populations MIC=1 MIC=2 MIC=4

34 Cefotaxime susceptibility testing in E. coli % isolates

35 What is antimicrobial resistance?

36 superbugs Microorganisms with multiply resistance MRSA - methicillin-resistant Staphylococcus aureus VISA VRE - vancomycin-resistant enterococci ESBLs - extended-spectrum beta-lactamases (microorganisms resistant to cephalosporins) vancomycin intermediate resistant Staphylococcі % Staphylococcus infections were cured by penicillin 1982 only 10 % infections At nowadays?... MRSA causes deaths annually in USA

37 What is antimicrobial resistance I? The ability of a microorganism to survive at a given concentration of an antimicrobial agent at which the wild type population of the microorganism would be killed This is called the microbiological breakpoint. EUCAST* is defining the microbiological breakpoints. *European Committee on Antimicrobial Susceptibility Testing

38 Population distribution 60 Resistant population (non-wild type population) = Number of isolates Sensitive population (wild type population) Break point >128 µg/ml MIC 0,25 0, MIC > Breakpoint! Resistant (R > 8 or R 16)